Refined Study on 3D Flow Characteristics around Bridge Piers

In order to investigate into the characteristics and mechanism of the current forces on bridge piers, a refined study on 3D flow around the typical pier model was conducted considering the influence of the free surface. ANSYS FLUENT was employed to address the characteristics of the drag force and lift force on the whole pier model. The pier model was divided into five sections from the bottom to top of the cylinder, characteristics of the drag and lift forces on each section were compared, and the variation law of the drag and lift forces along the water depth was analyzed. Further, the influences of the free surface and the bottom condition on vortex structure were analyzed and the relationship between 3D flow field and current force was discussed. Results show that the current force on pier is not uniformly distributed along the water depth; i.e., the mean value of drag force on cylinder sections c1−c4 accounts for 25%, 30%, 25% and 20%, respectively, of the total drag force on pier, and c5 contributes almost zero due to its exposure to air. The amplitudes of the drag and lift forces in the middle-lower part are larger than those at the bottom, middle-upper, and free surface parts of the water depth. Besides, the alternating vortex shedding causes alternating fluctuations of the free surface at the left and right sides of the cylinder. The free surface suppresses the vortex shedding, and vortices of different scales exist at the free surface, which are quite different from the two alternately shedding vortexes under the free surface. The vortex shedding at the middle-lower part of the pier lags behind the rest parts, resulting in a significant phase difference in the lift force at different parts of the cylinder. The lift force is comparable in magnitude to the average value of the drag force, for example which are respectively 5.511 N, 3.695 N in case 3, showing that the possible vibration of the pier or bridge caused by the lift force cannot be ignored.